1. Field of the Invention
The present invention relates to a sheet glass flattening method, a method of manufacturing products using a glass substrate produced using the flattening method, and the products themselves. More particularly, the present invention relates to a method of a manufacturing glass substrate for an information recording disk used for hard disks, optical recording medium, and similar things, and a glass substrate technique applicable advantageously to magnetic recording disks and the like manufactured by the above mentioned manufacturing method.
2. Description of the Related Art
Conventionally, thin glass substrates have been used for magnetic recording disks (hard disks), optical disks, liquid crystal displays, and the like. Recently, glass substrates have become popular because they have excellent characteristics such as higher flatness and sheet thinning capability, compared with other resin substrates and metal substrates.
Usually, the thin glass substrate is prepared by processing a sheet glass blank, obtained by fabrication through a method such as the down-drawing method (fusion method) or floating method, to a predetermined dimension and then abrading the surface of the result.
The down-drawing method is generally a method of manufacturing sheet glass by drawing down molten glass and then pulling down vertically the same. For example, the method can be done by having molten glass run down along the front and back surfaces of a molding body with a wedged cross section and then joined at the lower portion of the molding body in a sheet form. The sheet is then cooled and solidified by pulling down upon it with a pulling roller to produce sheet glass (see Japanese Unexamined Patent publication (Tokkai-Hei) No. 5-163032 or U.S. Pat. No. 3,338,696).
A sheet glass produced by the down-drawing method is thinner than that produced by other methods and can be used as a thin glass substrate for magnetic recording disks and for liquid crystal displays. However, because of the characteristics of this forming method, the flatness of the sheet is degraded compared with the sheet glass produced by the floating method. For that reason, where thin film glass produced by the down-drawing method is used for the above-mentioned applications, a heat treatment (heating and annealing) is needed to correct and improve the flatness of the sheet glass.
In the method, as shown in FIGS. 1(a) and 1(b), the heating and annealing are carried out with thin glass substrates 11 sandwiched between cut and polished densified sheets (thick, flat correcting aluminum sheets) 10 with a paper or carbon paper 12 acting as a lubricant inserted between either surface of the thin glass substrates 11 laminated and the densified sheet 10. Numeral 13 represents a fin for preventing the densified sheet from being deformed.
With plural thin glass sheets laminated to increase efficiency, and when the heat treatment is done without any consideration, the plural glass sheets are thermally bonded together, and completely damaged. Hence, it is necessary to prevent the thin glass sheets from becoming bonded together. To overcome this problem, one method inserts a paper or carbon paper between the thin glass sheets and utilizes paper cinders, or carbon paper as a lubricant (see Japanese Unexamined Patent Publication (Tokkai-hei) No. 6-247730).
Where paper is sandwiched between the thin glass sheets, as the number of laminated thin glass layers increases, the accuracy of the flatness cannot be accomplished to a desired value because of the paper's thickness and elasticity. Labor is also needed in handling and cleaning the cinders. In more detail, in the paper sandwiched between the thin glass sheets, the paper's peripheral portion is burnt out, but its center portion remains carbonized due to oxygen shortage so that there is a difference in thickness between the peripheral portion and the center portion of the thin glass sheet. The thickness is increased by the laminated value, and the flatness becomes poor due to the increased thickness transferred onto the thin glass sheet. This results in insufficient satisfaction to a required accuracy. Moreover, when the thin sheet glass is taken out of a heating furnace after the thermal treatment, paper cinders may be scattered around, thus degenerating the working environment. Thus, the handling is troublesome. Moreover, a tar component from the paper adhered on the surface of the thin glass sheet is troublesome and causes additional cleaning labor.
Where a carbon paper is sandwiched between the thin glass sheets, the thickness and elasticity of the carbon paper make it difficult to set the accuracy of flatness to a desired value. Further, the costly carbon paper causes an increase in cost. In more detail, since the carbon paper is 0.5 mm or more thicker than paper, the elasticity absorbs the flatness of the sheet glass. As a result, the insufficient correction of the flatness causes the improved flatness accuracy. The carbon paper also leads to higher costs because of its insufficient strength and its brittleness.
In order to avoid the above-mentioned problems, fine powders of an inorganic material may be used as the lubricant. However, it is difficult to uniformly scatter (coat) the fine powders over the surface of a thin glass sheet, and the fine powders end up being scattered, thus degenerating the working environment. Moreover, since the fine powders fuse on the surface of a thin glass sheet or hurt the surface thereof, they deteriorate the abrasion property of the thin glass sheet to an unusable state.
In addition, when paper or carbon paper is inserted between the thin glass sheets, dusts in air are involved. This causes flaws due to rubbing, thus resulting in a decrease in yield.
Because of such problems, it has been difficult to treat efficiently and thermally the laminated plural thin glass sheets with the flatness of a required accuracy, without producing flaws and foreign matter bonding.
For that reason, either at cost of the efficiency, a heat treatment is performed with the thin glass sheets respectively sandwiched between densified sheets with good flatness to secure the flatness of a required accuracy, or at the cost of the accuracy, a heat treatment is performed with paper or carbon paper sandwiched between thin glass sheets. Hence, the cinders are troublesome while cleaning is unavoidable.
In the conventional sheet glass flattening method, it has also been difficult preventing the laminated thin glass sheets from being bonded. In this method, it is difficult to obtain the flatness, with a required accuracy by treating efficiently and thermally the laminated plural thin glass sheets, without producing flaws and foreign matter bonding.
One problem is that a conventional glass substrate is costly because either the flatness with an accuracy required at the cost of efficiency must be secured, or a thin glass sheet prepared at the cost of the accuracy requires considerable polishing to secure the flatness of required accuracy.
Previously, aluminum substrates have been used as the substrate for magnetic recording disks. However, with the demands for small size, thin magnetic recording disks and low-floating of a magnetic head, glass substrates are being used at an increasing rate because small-sizing and thinning, high flatness, and low floating over a magnetic recording disk are more easily realized with the glass substrates as compared with the aluminum substrates.
However, a magnetic recording disk glass substrate of that kind is costly, and the surface condition of the substrate is poor because flaws unremovable in the polishing step and foreign matter boding are left on the substrate. Hence, there has been a problem in that a head crush occurs when the substrate is used for a magnetic recording disk, or defects occur in a film such as a magnetic layer, thus causing an error.
Where a glass substrate is used as a magnetic recording disk substrate, the surface of the glass substrate is generally subjected to a chemical strengthening process by a low-temperature ion exchanging method to improve the shock resistance and vibration resistance.
This chemical strengthening is make after cutting and polishing the glass substrate. There is also a substrate cleaning step prior to the chemical reinforcement process. This cleaning step is carried out with pure water, as described in Japanese Unexamined Patent Publication (Tokkai-Hei) No. 2-285508.
Conventionally, the polishing is made to the front and back surfaces of a glass substrate, but is not made to the outer peripheral end surface and inner peripheral end surface. Since these ends have rough surfaces, dusts may occur from the ends when unintentionally rubbed when the glass substrate is carried, or stored in or taken out of a housing case.
Recently, the slight dusting caused problems with the high density magnetic recording disks.
It is, however, possible to polish the end surfaces. Unfortunately, this leads to a high cost because it is difficult to polish the end surface to the surface roughness with which dusting can be prevented. It is also possible to polish by chemically etching the ends. However, the strong etching effect may make it difficult to obtain end surfaces with good surface roughness. Particularly, in the case of the glass substrate used for magnetic recording disks, the roundness is impaired through the etching process so that the center deviation results in rejected products. The strong etching effect may even reversely deteriorate the surface roughness of the end surface.
As stated supra, where the glass substrate is used as a magnetic recording disk substrate, the surface of the glass substrate is generally subjected to a chemical strengthening process.
The method disclosed in Japanese Unexamined Patent Publication (Tokkai-Hei) No. 5-32431 is a known method of manufacturing a magnetic recording disk using that kind of chemical reinforced glass substrate. According to the method described in the publication, the glass substrate is reinforced by immersing it into a mixed solution of potassium nitrate and sodium nitrate at 400 C., and an underlayer and a magnetic layer are successively formed on the chemical reinforced glass substrate.
However, as described in the conventional method, in the magnetic recording disk, which is manufactured by pulling up a glass substrate from the chemical reinforcement solution, cleaning it, and then successively forming an underlayer and a magnetic layer on the glass substrate, there is a disadvantage in that abnormal protrusions sometimes occur on the surface of the magnetic recording disk. As described above, the abnormal protrusions formed on the surface of a magnetic recording disk may cause a head crush to a magnetic head so that the magnetic recording disk cannot be used as a product.
Since molten salt adheres to the glass substrate after the chemical strengthening process, cleaning is required. Conventionally, Japanese Unexamined Patent Publication (Tokkai-Hei) No. 2-285508, for example, discloses a magnetic recording disk glass substrate after the chemical reinforcement process is cleaned using alkaline cleaning agent, pure water, and organic cleaning agent.
With the high recording density of a magnetic recording disk, it has been required to decrease the distance (spacing) between the magnetic recording disk and the magnetic head. Hence, the current problem is to remove completely the foreign matters on a glass substrate causing protrusions formed on the surface of a magnetic recording disk.
However, the above-described conventional cleaning method can provide a cleaning effect to some extent. It has been, however, difficult to completely remove the molten salt left on the glass substrate even if the glass substrate is cleaned after being pulled up from the chemical reinforcement solution.